I still think that 280 lbs of damage is too little. The amount of devastation these 28cm rockets caused should be reflected, just the amount of TNT in the warheads alone vs all other things worth of comparison in AH should give these things the ability to knock down the typical OBJ in 1 shot.
The 28cm Wurf-Körper-Spreng rocket had a mass of 82kg and a warhead of 50kg, with a maximum range of 1,925 meters. The 30cm Wurf-Körper 42 Spreng rocket had a mass of 125.7kg and a warhead of 44.66kg, with a maximum range of 4,550 meters. The 32cm Wurf-Körper Flamm rocket had a mass of 79kg, with a maximum range of 2,200 meters; its warhead was 50 liters of oil and a 1.6kg primer, and could cover 200 square meters of ground with burning oil.